This invention pertains to transmission systems and more particularly to high-voltage transmission systems particularly useful for transmitting electricity over great distances at low cost.
DC transmission is known to be significantly less costly than AC, but heretofore has been limited by the high cost of conversion involving the necessary transformers and filtering devices. In view of the present development of energy sources such as nuclear, solar, coal mine mouth generation, etc. usually located at great distances from high density urban areas where the electrical energy will be consumed, DC transmission now appears highly desirable as the means to convey the power produced by these sources.
In well known high-voltage DC generation systems a 60 Hz or other low frequency, low-voltage generator is coupled to a high-voltage step-up transformer. The output of the transformer is rectified and transmitted via a number of filters. The term "high voltage" as used herein pertains substantially to at least 138 kV.
In high-voltage DC transmission systems of the kind described substantial filtering is typically required, at significant expense, to remove the ripple superimposed at the DC output. Substantial filtering on the primary side of the transformer is also required to prevent surge and harmonics from getting back to the generator.
As is known, an increase in frequency reduces the degree of ripple and of filtering required. Hence, it is desired to generate at high-voltage and at high frequency at least of the order of 120 Hz in order to reduce the filtering requirements and attendant expense. Since, as noted above, the usual high-voltage DC system employs a low frequency, low-voltage generator coupled to a high-voltage transformer, the frequency of the output of the transformer remains at low frequency with attendant burdensome ripple requiring filtering.
The mere substitution of a low-voltage high-frequency generator in the above type of well known system is deemed unsatisfactory since increasing the frequency of the geneator increases the transformer impedance and losses (especially core losses), and decreases the power transmitted. The transformer reactance (caused by leakage flux) and hence the reactance voltage drop varies in direct relation to the frequency.
Accordingly, doubling the frequency provides a distinct disadvantage in the provision of prior DC power stations as above. Furthermore, it is impractical to try to generate at substantially higher voltages and frequencies with a conventional generator in view of the fact that the flux density is limited by iron saturation, and the armature turns must be insulated from the grounded iron thereby limiting the turns density and the voltage.
As is known, the amplitude of the ripple is decreased approximately inversely as the square of the number of phases. Similarly, the filters are reduced as the frequency is increased. In a conventional system utilizing a high voltage transformer, as the number of phases are increased, the cost of the transformer(s) increases in direct proportion, which cost is substantial. Whereas by employing a superconducting high-voltage generator system herein disclosed, the transformer(s) is entirely eliminated. This permits an increase in the number of phases without incurring this cost penalty.
It has been observed that in a conventional generator, iron is essential in order to lower the reluctance of the magnetic circuit, but limits the flux density and the output voltage. On the other hand a superconducting generator eliminates most of the iron so as to introduce into the generator degrees of freedom not previously obtainable. Accordinly, it is possible (with a superconducting generator) to generate at full-line voltage (up to 500 kV and higher). This is possible in view of the fact that when grounded iron is removed, only the inter-turn voltage needs to be insulated between adjacent bars throughout most of the armature. This further eliminates the need for a high-voltage transformer and by increasing the frequency and number of phases reduces the filtering requirements substantially.
Elimination of the transformer accomplishes a reduction in the capital and operating expenses involved in same while the higher voltage in the superconducting generator is achieved at almost no increase in cost. In addition by eliminating the transformer and employing a generator which can generate at high frequency and voltage a given (capacitance/inductance) filter becomes more effective in filtering out the ripple superimposed on the DC. Therefore less reactance is needed in the filters both on the input and output sides of the diodes (rectifying means).
Thus, by removing the transformer the higher frequency becomes a decided advantage. Reactors, including inductance and capacitance, can be eliminated substantially in the ratio of the new frequency to 60 Hz. In addition the reactors present on the primary side of the transformer to protect the generator from surge and harmonics can also be eliminated.
Aside from a safe factor of about 20% overspeed, the frequency of a generator cannot usually be expected to be increased by simply running it at a higher speed since turbo-generator sets are generally designed for optimum performance. This requires them to operate near the centripetal force stress limit of the rotor materials. However, in a superconducting generator, the number of rotor poles may easily be increased to increase the frequnecy. (Similarly, the number of phases may be easily increased.) Thus, in an AC generator of a given rating operating at 3600 rpm and 60 Hz, if the rotor is changed from 2 to 4 poles the output frequency will increase to 120 Hz while it continues to run at the same speed. Similarly, if the rotor is increased to a 6 pole rotor the frequency will increase to 180 Hz.
Reactor devices characterized by inductance and capacitance are employed for effecting the requisite filtering on the output as needed. The need becomes much less as the frequency or the number of phases increases, as noted above. An increase in frequency by a factor of three can reduce the reactor requirement by substantially a factor of three. An increase in the number of phases by a factor of two reduces the reactor requirement by approximately a factor of four.
A detailed description of high-voltage superconducting generators is given in EPRI Report RP429-1, November, 1977, Superconducting Generator Design, for which Mario Rabinowitz was the EPRI Project Manager. Additionally, the following two-pre-prints prepared to be presented at the IEEE Winter Power Meeting, January, 1978, and now available also describe the design and operation of a high-voltage superconducting generator: 1. New Armature Winding Concepts for EHV (Extra High Voltage) and High CFCT (Critical Fault Clearing Times) Applications of Superconducting Turbine Generators, by C. Flick. 2. Design of Large Superconducting Turbine Generators for Electric Utility Application, by J. H. Parker, Jr. and R. A. Towne.
A third published paper also discusses this, entitled Cryogenic Power Generation, by Mario Rabinowitz, Cryogenics, Vol. 17, p. 319-330, June, 1977.